Frictional retardation of laminar flow by plant litter and surface stones on dryland surfaces: A laboratory study

The relative contributions of plant litter and surface stones to the frictional retardation of laminar flows were investigated in a series of laboratory experiments on a glued sand board. A computer-controlled measuring gantry was used for precise recording of flow depths, and retardation was quantified using the Darcy-Weisbach friction factor f. Surface stones had relatively little effect on flow retardation or flow speeds but increased flow depths in comparison with the satire discharge across a stone-free surface. In contrast, plant litter produced larger reductions in flow speed and increases in flow depth, associated with large increases in flow retardation. These effects were most marked at the highest loading tested, 80 g m(-2). Results show that the retardation caused by a given surface cover fraction of litter significantly exceeds that caused by the same cover of surface stones, emphasising the need for measures other than surface cover to be employed in work on overland flow. Neither nonsubmerged obstacles such as surface stones nor plant litter contribute increased roughness with rising discharge in the laminar regime. Instead, values off decline as Reynolds number rises. These findings contrast notably with relationships established for turbulent flow and suggest that for correct modeling of overland flow in drylands, the temporal and spatial ranges of laminar flows must be distinguished from those where turbulent flows occur.